She went into Tuesday's primaries with a commanding lead of nearly 300 pledged delegates over Mr Sanders, the democratic socialist senator from Vermont.

She also had a dominant advantage among party officials and elected leaders known as super-delegates.

Mrs Clinton remains on track to clinch the Democratic Party's presidential nomination but she is trying to avoid ending the primary season with a string of losses.

Mr Sanders has vowed to campaign through to the final primary in Washington DC on June 14.

All of the Democratic delegates are awarded proportionally, meaning the results in Kentucky and Oregon will do little to upset the race.

Mr Sanders's continued presence in the race is prompting concerns among Clinton allies that he will damage her ability to take on Donald Trump, the presumptive Republican presidential nominee, in the general election.

Tuesday's elections also took place amid new questions about party unity following a divisive weekend state party convention in Nevada.

Supporters of Mr Sanders tossed chairs and made death threats against the Nevada party chairwoman, arguing that the party leadership rigged results in favour of Mrs Clinton. The Nevada incident was a warning about the potential for disruption at July's Democratic National Convention in Philadelphia.

Republicans also went to the polls on Tuesday in Oregon where Mr Trump was the only candidate still in the race. In a sign of his pivot into the general election, his campaign announced that it had signed a joint fundraising agreement with the Republican National Committee that will allow it to raise cash for his campaign and other Republican efforts.

After months of discord within the 'Grand Old Party' (GOC), the Democratic Party has now displayed signs that it could have trouble uniting around Mrs Clinton's candidacy as Mr Sanders ploughs through the end of the primary calendar in mid-June. Sanders will need to win about two-thirds of the remaining pledged delegates to end the primary season in a tie but is not letting up.

Opportunity

"Before we will have the opportunity to defeat Donald Trump, we're going to have to defeat Secretary Clinton," Sanders said on Tuesday night to cheers in Carson, California.

Mrs Clinton ended the night with a commanding lead of 279 pledged delegates over Sanders and a dominant advantage among party officials and elected leaders known as superdelegates.

The outcomes in Kentucky and Oregon, where Sanders led by nine percentage points with roughly three-quarters of the vote counted, did not dramatically change the delegate count.

Former secretary of state Clinton remains on track to clinch the nomination on June 7 in the New Jersey primary.